The present invention relates generally to methods and apparatus for testing of electric motors and more specifically to methods and apparatus for verifying motor size and detecting miswiring of motor windings.
Manufacturers of induction motors and also manufacturers of products incorporating induction motors generally make or use motors of various sizes. For example, a manufacturer of refrigerator compressors typically produces several compressor models each having a different capacity and incorporating a different size motor. Similarly the manufacturer of refrigerators typically produces several different models incorporating different size compressors. Obviously, it is essential to proper performance of the final product that the motor be the correct size. This is of concern with respect to refrigerator compressors because the external housing of the compressor may be the same for compressors of different capacity, equipped with motors of different horse power ratings. Thus, a visual inspection of the assembled compressor is unreliable. Also, in a highly automated manufacturing environment visual inspection may not be practical. Furthermore, such inspection would not detect miswiring of the motor which may erroneously occur in manufacture.
Over the years many different methods have been used by compressor manufacturers as well as manufacturers of other products incorporating induction motors to identify a motor by making electrical measurements of its start and run windings. In accordance with one conventional approach to making resistance measurements of the motor windings, the resistance of each winding is individually measured and compared to an upper and lower reject limit. This technique has been found to be not totally satisfactory in practice because the absolute value of the winding resistance may vary widely from one production lot of motors of the same size to another. Also the absolute resistance values for windings may vary widely with temperature.
An alternative approach to motor identification testing has been to make power measurements. However, it is desirable to determine motor size when the motor has little or no loading. Power measurements present a problem because under no load conditions different size motors may consume similar amounts of power.
Thus, there is a need for a method of testing motors to verify correct motor size and to detect miswiring defects which is more reliable than previously known methods and which readily lends itself to use in a highly automated production environment.
It is, therefore, a primary object of the present invention to provide a method and apparatus for reliably verifying motor size and detecting miswiring defects which is highly compatible with automated assembly of the finished product incorporating the motor.